Gain control system for facsimile scanning



May 9, 1950 H. E. HAYNES GAIN CONTROL SYSTEM FOR FACSIMILE SCANNING INVENTOR [[aroldfifiz'aym BY ATTORNEY Filed Oct. 31, 1946 Patented May 9, 1950 GAIN CONTROL SYSTEM SCANNING FOR FACSIMILE Harold E. Haynes, Indianapolis, 'Ind-., assignor to Radio Corporation of America, a corporation of D aware Application October 31, 1945, Serial'No. 706,925

The present invention relates to systems for producing an image of scanned subject matter such as an object or a representation in the form of a picture or map and more particularly, but not necessarily exclusively, to a novel arrangement -for maintaining a consistent signal level in a facsimile scanning system.

In accordance with the invention, a recurring reference level of a varying quantity controls the sensitivity of one of the elements of the scanning system sothat response to this reference level is maintained; For example, in the embodiment seilected for illustrating the invention, a given amount of light, when scanning a copy area of a definite shade, should produce a definite output current in the scanning phototube and if this definite shade is sufficiently recurrent this amount of light can be caused to control the phototube sensitivity so as to produce the desired current.

Where the subject matter to be scanned is printed I or inscribed on a substantially white piece of record bearing material, such as paper, then white may be selected as the reference level. It will be understood, however, that the background may have other tints or shades, or that recurrentareas of the subject matter itself may serve for control purposesin accordance with the invention.

Accordingly, the principal object of the present invention is to provide a novel arrangement for maintaining a substantially fixed signal level in a facsimile scanning system.

Another object is to provide a novel signal level control which is maintained in operation by a recurrent condition of light intensity.

Aiurther object is to provide a novel signal level control embodying a ,phototube and-a power'supplyfor the phototnbe together with novel means for controlling thepoiver supply.

A still furtherobject is to provide novel means for obtaining a control voltagein a facsimile scanning system.

Other objects and advantages of the present invention will, of conrse, become apparent and immediately su gest themselves to th se skilled in the art to which the invention is irected rom a reading of t e f llowing sp c fi a i c nnection w th he a compa y ng dr w n wh h shows schematically ajacsimile system embodying the invention and including a transmitting scannerandarecording scanner.

The facsimilesystem shown in the single figure of the drawing is operative to scan wappropriate subject matter in ,a transmitting scanner indicated in its entirety by reference character It to produce image currents which serve to operate 5 l j S- Cl. 178-616) 2 a recording scanner 12 for the purpose of produce ing a recordedimage of the scanned subjectmat ter. The transmitting scanner It may be ofkany desired kind or type and is shown illustratively comprising a drum M on which is secured the subject matter copy it which bears a picture, printed matter or other indicia I! to be reptile duced by the recording scanner or, as it will be calledhereinafter, the recorder I2,

A facsimile signal generatingscanning head which carries a light source [8 and a phototube 20, preferably of: the electronmul-tipliertype, is propelled axially a-long'the drum l4 bya lead screw-2i. Any suitable change-speed device 22 may, if desired, be used to connect the drive shaft v23.- of a driving motor M with the driveshaft 2 6 of the scanner. A gear and pinion; combination 28. drives the lead screw 2] at the proper relative spcedso thatclosely spaced helical scanning lines are obtained as the scanning head including 13 B light source It moves axially of the drum 14. It will .be understood that. the scanning head includes a suitable optical. system for imaging a light .spot from the light source upon the surface of the drum Hi and hence upon the subject copy lfifififillllfid thereto. The optical system isshown, illustratively, as comprisin two lenses 3.! rand-.32.

T recorder 12 is mechan cal y driven from the transm tin anne th ugh a shaft .3 driv ins pow r b in impa ted to a drum 3 6. i he ecord r whic car ies ahelix 3,3. Thehel x ooperates with-a prin er ar 39 in a. Well k n m i re rd shect (no can t r d ce marks upon a shown) pas ng b twe n he-he ix and the print r ba A corder ct this general ty e wi h areco d s c p r d i e is sh wn Patent Re. 20,152, granted to C. J, Young on OQfiObfir 27,

19,36. The recorder in this last namedpatent re:-

.lies on transfer oi marking material to produce markings by vibrating the printer bar rather than on electrical or other processes which depend upon a direct application ,of electrical efiects to the record sheet. Other-details of a facsimile recorder employing an electrolytic processfor producing marks are shown in U. S. Patent No. 2,391,765; granted to Maurice Artzt on December 25, 1945;

The mechanical portions of the facsimile'systern so far described comprise what is usually termed a facsimile duplicator. However, it will be understood by those skilled in the art that the recorder 12 may 'belocated at a point which is remote from the transmitter and may be driven maintained in operation synchronously with the transmitterjby anywell known-synchronizing "tinued use and changes in exciter lamp voltage D. C. source I9.

tube 56 to the grids 82 and 83 .ondary Winding 92 which,

,conventionally by reference mitters which may be employed are shown in U. S. Patents Nos. 1,746,407 and 2,298,911, granted to F. Schroeter et al. and C. J. Young on February 11, 1930, and October 13, 1942, respectively.

In accordance with the invention an automatic gain control is disclosed which sets the over-all gain to such a value that the portions of the subject matter being copied which have the greatest -or highest reflectivity will be rendered as white by the recorder. That is, the gain value necessary for any piece of copy is automatically set by its whites or highest light reflecting part. The electron multiplier phototube is capable of very great sensitivity and it is in itself capable of operating from D. C. to the highest frequencies which are produced upon scanning the subject copy at the transmitter at high speed. The sen- 'sitivity 'of a, phototube of this type is of value designated E3.

E2 in the following description of the operation of the system.

The anodes 96 and 91' of the output tubes 86 and 81 are connected through the primary 98 of a coupling transformer 99 to a suitable source of positive voltage I!!! which may be the source of low voltage supply indicated by reference character 64. The voltage applied at this point is The screen grids I82 and I03 of the output tubes are connected to an adjustable source of positive voltage as indicated diagrammatically at Hit. The voltage applied at this point is designated E1 The secondary I98 of the transformer 99 is connected to the anodes III of a full wave demodulator tube I I4. The load circuit of this demodulator tube includes a series resistor H6, the helix 1 38 and the printer bar 39 of since the light level received from the scanned I copy fluctuates in value from zero to perhaps .001 lumen in value. There are several sources of gain variation in the system, chief among them being changes in electrode voltages on the multiplier, changes in multiplier sensitivity with conbrightness. The automatic gain control of this invention maintains'proper operating conditions in spite of g ain variations introduced from the sources just named while at the same time takes full advantage of the largegain due to the electron multiplier action.

The drawing shows, by way of example, a systerm. in which the output from the phototube 20 modulates a local oscillator signal which is gen- 'erated by an oscillator designated in its entirety by reference character 45. The oscillator 45 comprises the triode section 45 of a duplex vacuum tube 41 and an oscillator transformer 48. The latter, is made upof a grid coil 49 and a plate coil 1 5|. The anode 52 of the oscillator is coupled by way of a modulator tube 56. The signal grid 58 of of a capacitor 53 to the suppressor grid 54 the modulator tube 55 is connected to the anode 6| of the phototube 20. A grid resistor 69 is connected to a source of positive potential such, for

example, as one of the terminals of a D. C. supply source 64 or a suitable voltage divider therein, one end of which is grounded as indicated by reference character 66. The voltage applied. at this point is designated E6 in the following description. Other terminals shown on the draw- ,ing may be connected to the source 64.

The cathode 68 of the phototube 28 is connected to the negative terminal A coupling transformer BI serves to couple the output of the modulator of a pair of output tubes 85 and 81. The transformer 8| comprises a primary winding 88 which may, when desired, be tuned by a capacitance BI and aseclikewise, may, when desired, be tuned by a capacitance 93. The source of anode voltage for the tube 56is designated? character 94. The: voltage applied at this point will be designated of a suitable high the recorder I2. The helix 38 is connected to a suitable metallic part of the recorder I2 and is grounded as indicated at H8.

A tube I22 is connected in series with the high voltage D. C. supply source I9. The cathode I24 of this tube is connected to a source'of positive potential such, for example, as one of the terminals of the source 64 or suitable voltage divider therein. Adjustability of the Voltage designated E4 applied to the cathode I24 is indicated schematically at I26.

The triode section I28 of the previously mentioned tube 41 is employed as a diode by connecting its anode I29 to its grid I3 I The cathode I32 is connected to the cathodes I33 of the demodulator tube II4 which becomes the reference point for obtaining the control voltage. The resistor H6 prevents the reference point from going to ground potential in case of a bar-to-helix short circuit. The diode anode I29 of the tube section I28 is connected to the control grid I36 of the tube I22.

While tube 41 has been shown as a duplex tube with one section employed as a diode. it will be understood by those skilled in the art that a separate triode and diode may be used.

Added details of the system shown illustratively on the single figure of the drawing as well as the manner in which it functions will now be described in detail. The high voltage D. C. supply I9 and the low voltage D. C. supply 64 may be connected to a suitable power source such as A. C. mains I38.

The load resistor 63 is connected to E6 for contrast control. For normal contrast E6 is equal to zero. The voltage Ev applied to screen grid I39 of the modulator tube 56 is set so that when no light falls on the phototube 2n and with the grid bias approximately equal to zero, the plate current component at the carrier frequency flowing in the primary 88 of the transformer 8| produces enough signal at the grids 82 and 83 of the output tubes to produce black in the printing circuit which is driven by the demodulator H4. Thus, E7 is the black control and E6 is the contrast control. When current is drawn by the phototube 20 through the resistor 63, current to the printer bar 39 goes down approximately linearly as the phototube current goes up.

Automatic gain control inaccordance with the invention is accomplished by varing the total voltage supplied to the dynodes I4l of the phototube 20. The total voltage applied to the phototube 20 is adjusted in thepillustrative embodi- .ment by inserting the platecircuit of the tube I22 in series with the divider circuit across the multimodulator H4 as: a'reference-poi-nt as" explained above: When current is drawn by the phototube *through the resistor '63,; the current intheresistor-| It decreases: The voltage at the ref erence pointgoes more negative'and iiithe gain is toolgreat, thevqltage. at this point goessub stantially' tozero: The cathode I32 then goes more negativeand..currentissdrawn from an RC combination which-comprises a'resistor I42 and a capacitance I43 reducing thebiason thaslidIBG of the tube I 22 so as to increase the.resistanceuoii this tube and decrease the gain in the system.

Slightly-"morein detail, .tlieavoitage at the reference point, namely the demodulator fcathode I33, dips-toward zero withrespect to groundwhen Whiteds being "scanned by-photocell 20: If-the gain,- of the'system istoogreatjthis voltagerwill approach zero very closely butifthe gain isitoo' lpw,; it" will-not dip-so-near to-zeror These-minimum excursions of*the"voltageatthe reference point are indicative of whether gain setting is correct and are utilized by the diode section I28 of the tube 41 and the RC combination I 42 and I43 to adjust gain.

The cathode I24 of the tube I22 is operated at a D. 0. potential E4 which is approximately equal to the potential of the reference point when the desired white condition exists in the printing circuit, plus additional voltage which is the midrange bias on the tube 122. If the gain happens to be so low that the voltage at the reference point never drops as low as E4 then the potential of the grid I 36 will gradually tend toward E4, decreasing the resistance constituted by the plate current of the tube I22 and increasing the voltage on the multiplier of the photocell 20 which has the affect of increasing the gain in the system. Somewhere in the slow drift toward effective zero bias on the grid I36 of the tube I22, the system gain will have risen sufliciently that on white portions of the copy I6, the voltage at the reference point will dip below E4 and current will be drawn by the diode section I28 of the tube 4'! from the RC combination I42 and I43 tending to reduce gain. An equilibrium will be reached with white portions of the copy in the recorder I2 being given the desired white value. Changing E4 will adjust the minimum excursions of output current and is thus a white control.

A capacitor I46 is connected from ground to the positive side of the high voltage supply. Oscillation is prevented by having increased timing of the RC combination I 42-443 to decrease the grid circuit resistance of the tube I22.

Solely, by way of example, the following types of tubes may be employed in the illustrative system shown by the drawing. The phototube is a nine-stage electrostatically focussed type, the type 93l-A being suitable. In the type of phototube mentioned by way of example, the voltage across the terminal of the source 19 is to be approximately 1300 volts. The output tubes 86 and 81' may be type 807 tubes. The demodulator tube II4 may be a type 5V4G. Thetube I22 which is in series with a high voltage power supply may be a type 807. The modulator-tube may be a type 6AC'7. The combined tube 41 may be a type GSN'YGT. With the tube complement indicated by way of example, the voltage E1 may be 300 volts, the voltage E2 may be 250 volts, and the voltage Ea may be 600 volts. With the suggested tube complement; the-resistor- I42 may haves-is value of 220,000*ohms,- and:the ca pacitance- H52 may havea valueof 1 mi. The frequencvsofithlr. carrier generated" by the modulator: -5' B may; fort example; be 400 kilocycles. The side-bands; arising from themodulation process may for: ex-a:

ample; be plusor minus 15 kil'ocycles butthe bandwidth will vary with the: speed' ot scanningr and other factors.

Having nowdescribed the invention; whatzi'isz claimed and desired to -be secured by: Letters: Patent-is the following-z I; Facsimileapparatus for deriving facsimilee signals varying-in accordance with the l-ight andi; shade values of subject copy comprising-"means? fordeliveringdight varying in accordancewith the light and shade-values of subject copy to amelectron multiplier"phototube-in wh'ioh an elec-( trical current varies" in accordance with" the in tensityof received; light; meansfor generatin'g acarrier wave, meansformodulating said' carrier Wave in accordance: with the current in--said phototube, andmeans for-automaticallysetting the gain of the electronmultiplier of-said photo-- tube inaccordance-with a single light valuez 2. Facsimile apparatus for deriving facsimile signals varying in accordance with the light and shade values of subject copy comprising means for delivering light varying in accordance with the light and shade values of subject copy to an electron multiplier phototube in which an electrical current varies in accordance with the intensity of received light, means for generating a carrier wave, means for modulating said carrier wave in accordance with the current in said phototube, means for amplifying the modulated wave, means for demodulating the amplified wave, means for deriving a control voltage from the demodulating means, and means for automatically setting the gain of said phototube in accordance with the control voltage.

3. Facsimile apparatus for deriving facsimile signals varying in accordance with the light and shade values of subject copy comprising, means for delivering light varying in accordance with the light and shade values of subject copy to a sensitively responsive device in which electrical current varies in accordance with the intensity of light received by said device, means for generating a carrier wave, means for modulating the carrier wave in accordance with the current in said device, means for demodulating said carrier wave, recording means actuated by said demodulated carrier wave, and means connected to said recording means for setting the overall gain of said facsimile apparatus.

4. Facsimile apparatus for deriving facsimile signals varying in accordance with the light and shade values of subject copy comprising, means for delivering light varying in accordance with the light and shade values of subject copy to a. sensitively responsive device in which an electrical current varies in accordance with the intensity of light received by said device, recording means, means to provide signalling current to said recorder varying in accordance with the amplified output of said device, a resistor in the input circuit to said recorder to prevent total short-circuiting by the recorder of the output of said signal current providing means for said recorder, and means for automatically setting the overall gain of said facsimile apparatus, said last named means being responsive to a voltage representative of a predetermined light value.

5. A facsimile signalling system comprising a.

scanner having means including an electron multiplier phototube for generating signals representing the scanning of subject matter, an output circuit for said phototube including a resistor, means for generating a carrier frequency, a modulator tube comprising a cathode, ananode and a control electrode, said control electrode being coupled to said phototube output circuit, means coupling the output of said carrier frequency generating means to said modulator tube, a demodulator, diode rectifying elements coupled to the output circuit of said demodulator, a time constant circuit comprising a resistor and-a capacitor in parallel combination, a connection from the anode of said diode to one terminal of said capacitor and resistor combination, a control tube comprising a cathode, an anode andqa control'grid, a connection from said control grid to the anode of said rectifier section, a connection from the other terminal of said capacitor and resistor combination to the cathode of said control tube, a high voltage supplysourceior the electron multiplier section of said phototube, the cathode-anode circuit of said control tube being included in series between said electron multiplier section and said power supply source, means to apply a biasing voltage to the cathode of said control tube, said biasing voltage being approximately equal to the voltage developed in the output circuit of said demodulator whereby when maximum light falls on said phototube from the subject copy, current will be drawn-by said rectifier to increase the impedance of said control tube thereby to reduce gain.

HAROLD E. HAYNES.

,- REFERENCES CITED The following references are of record in the file 'of this patent UNITED STATES PATENTS Number Name 

